Chemical and particulate filters containing chemically modified carbon nanotube structures

ABSTRACT

A carbon nanotube filter, a use for a carbon nanotube filter and a method of forming a carbon nanotube filter. The method including (a) providing a carbon source and a carbon nanotube catalyst; (b) growing carbon nanotubes by reacting the carbon source with the nanotube catalyst; (c) forming chemically active carbon nanotubes by forming a chemically active layer on the carbon nanotubes or forming chemically reactive groups on sidewalls of the carbon nanotubes; and (d) placing the chemically active nanotubes in a filter housing.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to the field of chemical and particulatefilters; more specifically, it relates to chemical and particulatefilters containing chemically modified carbon nanotube structures andmethods of making same.

In advanced semiconductor manufacturing, airborne contaminants can causedegradation of photoresist layers and optical elements of advancedphotolithography systems such as immersion lithography tools, whereinairborne molecules can polymerize when exposed to the very high energylight beams of advanced lithography tools. The resultant polymer canthen coat the optics degrading the image quality of the tool and coatthe tooling causing degraded alignment tolerances. Additionallycontaminant molecules can be adsorbed by the photoresist layer,interfere with the photochemistry and cause photoresist defects.Conventional filters are unable to remove much of these airbornemolecules. Similarly, contaminant molecules can exist in the gas streamsused for purging and operating of various components of the tool.

Therefore there is a need for an advanced chemical and particulatefilter for applications requiring extremely low levels of contaminantsin the filtered air and/or gas streams.

SUMMARY OF THE INVENTION

The present invention utilizes carbon nanotubes having a chemicallyactive layer or carbon nanotubes having chemically reactive groups onthe sidewalls of the carbon nanotubes as a filter media. The small sizeof carbon nanotubes provides a large surface area and the chemicallyactive layer or chemically reactive groups provides sites forattracting, binding or chemically reacting with contaminant molecules inthe air or gas streams being filtered.

A first aspect of the present invention is a method of forming a carbonnanotube filter, comprising: (a) providing a carbon source and a carbonnanotube catalyst; (b) growing carbon nanotubes by reacting the carbonsource with the nanotube catalyst; (c) forming chemically active carbonnanotubes by forming a chemically active layer on the carbon nanotubesor forming chemically reactive groups on sidewalls of the carbonnanotubes; and (d) placing the chemically active nanotubes in a filterhousing.

A second aspect of the present invention is a filter, comprising: afilter housing; and chemically active carbon nanotubes within the filterhousing, the chemically active carbon nanotubes comprising a chemicallyactive layer formed on carbon nanotubes or comprising chemicallyreactive groups on sidewalls of the carbon nanotubes.

A third aspect of the present invention is a filter, comprising: afilter housing; and chemically active carbon nanotubes within the filterhousing, the chemically active carbon nanotubes comprising a chemicallyactive layer formed on carbon nanotubes or comprising chemicallyreactive groups on sidewalls of the carbon nanotubes; and mediacontaining the chemically active carbon nanotubes.

A fourth aspect of the present invention is an immersion exposure systemfor exposing a photoresist layer on a top surface of a wafer to light,comprising: an environment chamber containing a light source, one ormore focusing lenses, a mask holder, a slit, an immersion head and awafer stage, the light source, the one or more focusing lenses, the maskholder, the slit, and the immersion head aligned to an optical axis, thewafer stage moveable in two different orthogonal directions, each theorthogonal direction orthogonal to the optical axis, the mask holder andthe slit moveable in one of the two orthogonal directions, the immersionhead having a chamber having a flat top, a sidewall and a bottomopening, the flat top transparent to selected wavelengths of light;means for filling the chamber of the immersion head with an immersionliquid, the chamber of the immersion head aligned to the optical axis; afilter in a sidewall of the environment chamber, the filter comprising:a filter housing; and chemically active carbon nanotubes within thefilter housing, the chemically active carbon nanotubes comprising achemically active layer formed on carbon nanotubes or comprisingchemically reactive groups on sidewalls of the carbon nanotubes; andmeans for forcing air or inert gas first through the filter then intothe environment chamber and then out of the environment chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of an illustrative embodiment when readin conjunction with the accompanying drawings, wherein:

FIGS. 1A through 1E are cross-sectional views illustrating a firstmethod of making carbon nanotubes;

FIGS. 2A through 2E are cross-sectional views illustrating a secondmethod of making carbon nanotubes;

FIG. 3 is a isometric view of carbon nanotubes made by the methodsillustrated in FIGS. 1A through 1D and 2A through 2D;

FIG. 4A is cross-section view illustrating one process step of a thirdmethod for making carbon nanotubes;

FIG. 4B is an end view and FIG. 4C is a cross-sectional view throughline 4C-4C of FIG. 4B of nanotubes made by the third method of makingcarbon nanotubes;

FIG. 5 is a schematic drawing of an apparatus for making carbonnanotubes according to a fourth and fifth method;

FIG. 6A is a cross-section view and FIG. 6B is a cross-section viewthrough line 6B-6B of FIG. 6A of a first exemplary chemically activenanotube filter according to the present invention;

FIG. 7A is a cross-section view and FIG. 7B is a cross-section viewthrough line 7B-7B of FIG. 7A of a second exemplary chemically activenanotube filter according to the present invention;

FIG. 8A is a cross-section view and FIG. 8B is a cross-section viewthrough line 8B-8B of FIG. 8A of a third exemplary chemically activenanotube filter according to the present invention;

FIG. 8C, is an extension of the third exemplary chemically activenanotube filter of FIGS. 8A and 8B;

FIG. 9A is a cross-section view and FIG. 9B is a cross-section viewthrough line 9B-9B of FIG. 9A of a fourth exemplary chemically activenanotube filter according to the present invention;

FIG. 10A is a cross-section view and FIG. 10B is a cross-section viewthrough line 10B-10B of FIG. 10A of a fifth exemplary chemically activenanotube filter according to the present invention;

FIG. 11A is a cross-section view and FIG. 11B is a cross-section viewthrough line 11B-11B of FIG. 11A of a sixth exemplary chemically activenanotube filter according to the present invention;

FIG. 12 is a cross-section view of a modified high efficiencyparticulate air filter according to the present invention;

FIG. 13 is a flowchart of the method of making chemically activenanotube filters according to the present invention; and

FIG. 14 is a pictorial representation of an exemplary immersionlithography system incorporating a chemically active nanotube air filteraccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Carbon nanotubes are more correctly called carbon fullerenes, which areclosed-cage molecules composed of sp²-hybridized carbon atoms arrangedin hexagons and pentagons. There are two types of carbon fullerenes,namely closed spheroid cage fullerenes also called “bucky balls” andfullerene tubes. Fullerene tubes come in two types, single wallfullerenes tubes, which are hollow tube like structures or andmulti-wall fullerene tubes. Multi-wall fullerenes resemble sets ofconcentric cylinders. The present invention utilizes single-wall carbonfullerenes, hereinafter called single-wall nanotubes (SWNT) andmulti-wall carbon fullerenes, hereafter called multi-wall nanotubes(MWNT). For the purposes of the present invention, the term carbonnanotube (CNT) denotes either a carbon SWNT or a carbon MWNT.

The term chemically active nanotube filter refers to a filter containingcarbon nanotubes having a chemically active layer as a filter media orcarbon nanotubes having chemically reactive groups on the sidewalls ofthe carbon nanotubes as the filter media.

FIGS. 1A through 1E are cross-sectional views illustrating a firstmethod of making CNTs. In FIG. 1A, a substrate 100 is provided.Substrate 100 (or a uppermost layer on the substrate) is formed from amaterial that does not allow growth of a catalytic layer on the surfaceof substrate 100, so growth of CNTs on the surface of the substrate cannot occur. Note, as described infra, the catalyst itself (in oneexample, Fe (iron) atoms) is supplied from a gas stream. In one examplesubstrate 100 is a silicon substrate. Examples of other suitablesubstrates include substrates formed from, ceramic, metal, glass,plastic or having an upper layer of polysilicon, copper, gold, glass, orplastic.

In FIG. 1B, a template layer 105 is formed on substrate 100. Templatelayer 105 is formed from a material that allows formation of a catalyticlayer on the surface of template layer 100. It is this catalytic layerthat catalyzes growth of nanotubes on the surface of the template layer.In one example template layer 105 is silicon dioxide. Examples of othersuitable template layers include silicon oxy-nitride, aluminum oxide,magnesium oxide, and indium-tin oxide.

In FIG. 1C, template layer 105 (see FIG. 1B) is patterned into templateislands 110. Template islands 110 may also be considered a patternedcatalytic layer. This may be performed, for example by photolithographicprocess to form protective photoresist islands on top of template layer105, etching away the template layer where the template layer is notprotected by photoresist islands to expose the substrate and thenremoving the protective photoresist islands.

Alternatively, the processes described in relation to FIGS. 1B and 1Cmay be replaced by evaporation or deposition of template islands 110through a shadow mask onto substrate 100. An example of a shadow mask isa metal mask having a pattern of through holes. Evaporation ordeposition species can pass through the holes and deposit on thesubstrate. Where there is no hole, the evaporated species is depositedon the shadow mask.

In another alternative, the template layer is not patterned, the entiresurface of template layer 105 becoming one large template island 110.

In FIG. 1D, bundles of CNTs 115 are grown on template islands 110 byexposing substrate 100 and template islands 110 to a vapor mixture of aCNT precursor and a CNT catalyst at an elevated temperature. In oneexample, the CNT precursor is a xylene or xylene isomer mixture (C₈H₁₀)and the CNT catalyst is ferrocene (Fe(C₅H₅)₂) heated to between about600° C. to about 1100° C. Bundles of CNTs 115 take the shape of templateislands 110. If template islands 110 are circular a cylindrical bundleof CNTs (having a circular cross-section) will result. If templateislands 110 are rectangular, a rectangular bundle of CNTs with arectangular cross-section will result. Bundles of CNTs 115 have a lengthL1 and a width W1. In one example L1 is between about 100 microns about500 microns and W1 is about 10 microns to about 50 nm. In one example,the individual CNTs of each bundle of CNTs 115 formed by this firstmethod are predominantly MWNTs having diameters of between about 10 Åand about 2000 Å.

A more detailed discussion of formation of CNTs according to the firstmethod of forming CNTs may be found in United States Patent PublicationUS 2003/0165418 to Ajayan et al., filed on Feb. 11, 2003, which ishereby incorporated by reference in its entity.

In FIG. 1E, a chemically active layer 120 is formed on bundles of CNTs115 and then substrate 100 with attached bundles of CNTs is packagedinto filters. Formation of chemically active layer 120 comprises forminga chemically active layer on CNTs in each bundle of CNTs 115 or formingchemically reactive groups on the sidewalls of CNT in each bundles ofCNTs 115. Examples of chemically active layers include layers containingosmium dioxide (OsO₂), platinum (Pt), titanium (Ti), nickel (Ni), gold(Au), palladium (Pd), aluminum (Al) layers, Fe, or silicon oxides(SiO_(x)). Examples of chemically active groups include alkyl groups,aryl groups, fluoro groups, pyrrolidine groups, hydrogen, amino,aldehyde, carboxylate, amido, imino, and sulfonic groups. Forming ofchemically active layer 120 is discussed infra in more detail.

In an alternative version of the first method of forming CNTs describedsupra, the steps of patterning template layer 105 (see FIG. 1B) intotemplate islands are not performed and a random array of CNTs will beproduced.

FIGS. 2A through 2E are cross-sectional views illustrating a secondmethod of making CNTs. In FIG. 2A, a substrate 125 is provided.Substrate 125 (or a uppermost layer on the substrate) is formed from amaterial that is treated to form a nanoporous surface layer 130. In oneexample substrate 125 is a silicon substrate and nanoporous layer 130comprises an upper nanoporous layer having a pore size of about 3 nm ontop of a lower nanoporous layer having a pore size of about 300 nm. Inone example, when substrate 125 comprises silicon with a <100> crystalplane orientation, nanoporous layer 130 may be formed by electrochemicaletching of the surface of substrate 125 in an ethanol, hydrofluoric acidmixture.

In FIG. 2B, template islands 135 are formed on nanoporous layer 130. Inone example template islands 135 are formed from iron by evaporationthrough a shadow mask.

In FIG. 2C, substrate 125, nanoporous layer 130 and template islands 135(see FIG. 2B) are oxidized forming catalytic template islands 140 fromthe template islands. Any portion of the surface of nanoporous layer 135not protected by a catalytic template island 140 is converted to asilicon dioxide layer 145. In the example that templates islands 135(see FIG. 2B) are iron, then catalytic template islands 140 compriseiron oxide. Iron oxide is a material that allows (catalyzes) growth ofnanotubes on its surface. Therefore, catalytic template islands 140 mayalso be considered a patterned catalytic layer.

In FIG. 2D, CNT bundles 150 (each CNT bundle containing a multiplicityof individual CNTs) are grown on catalytic template islands 140 byexposing substrate 125 and catalytic template islands 140 to a CNTprecursor vapor a at an elevated temperature. In one example, the CNTprecursor is ethylene heated to about 700° C. If catalytic templateislands 140 are circular, cylindrical CNTs bundles (having a circularcross-section) will result. If catalytic template islands 140 arerectangular, CNT bundles with a rectangular cross-section will result.CNT bundles 150 have a length L2 and a width W2. In one example L2 isbetween about 30 microns about 250 microns and W2 is about 2 microns toabout 50 microns.

A more detailed discussion of formation of CNTs according to the secondmethod may be found U.S. Pat. No. 6,232,706 to Dai et al., filed on Nov.12, 1998, which is hereby incorporated by reference in its entity.

In FIG. 2E, chemically active layer 120 is formed on CNTs within eachCNT bundle 150 and then substrate 125 with attached CNTs is packagedinto filters. Again, forming of chemically active layer 120 is discussedinfra in more detail.

In a first alternative version of the second method of forming CNTsdescribed supra, instead of depositing iron through a shadow mask, ablanket iron layer is deposited and a random array of CNT bundles willbe produced. A blanket layer of iron may be deposited by evaporation orby spinning a concentrated iron salt solution onto the substrate andevaporating off the solvent.

In a second alternative version of the second method of forming CNTsdescribed supra, instead of using a porous substrate a catalytic layeror patterned catalytic layer is formed directly on a substrate such asquartz, ceramics, alumina, sapphire and silica.

FIG. 3 is an isometric view of CNTs made by the methods illustrated inFIGS. 1A through 1D and 2A through 2D. In FIG. 3, formed on a substrate160 are islands 165. Grown on islands 165 are CNTs 170. Substrate 160represents either substrate 100 (see FIG. 1A) or substrate 125 (see FIG.2A). Islands 165 represent either template islands 110 (see FIG. 1C) orcatalytic template islands 140 (see FIG. 2C). CNTs 170 represent CNTs110 (see FIG. 2C) or CNT bundles 150 (see FIG. 2D). CNTs 170 are spacedin rows and columns with rows spaced a distance S1 apart and columnsspaced a distance S2 apart. CNTs 170 have a height H1. Because thespaces S1 and S2 can be selected during manufacturing and height H1controlled during manufacturing process, spaces S1 and S2 and height H1can be selected to provide, first, sufficient space to allow room forfunctional groups to be attached to CNTs 170 and to provide the mostefficient spacing between CNTs with functional groups attached forattracting and capturing airborne contaminants or contaminants in gasstreams.

FIG. 4A is cross-section view illustrating one process step of a thirdmethod for making carbon nanotubes. The third method of the presentinvention utilizes the processes described supra for the first andsecond methods of the present invention except the substrate is a hollowcylinder instead of a flat substrate. In FIG. 4A, a cylindricalsubstrate 175 has a longitudinal axis 180 extending into and out of theplane of the paper. A cylindrical shadow mask 185 having a pattern ofopenings 190 is positioned between longitudinal axis 180 and an innersurface 195 of substrate 175 and catalytic islands 200 formed byevaporation or deposition (for example chemical vapor deposition (CVD))through opening 190 in shadow mask 185. The shadow mask is then removedand CNTs grown on catalytic islands 200 using either of the first orsecond methods described supra or other methods known in the art.

FIG. 4B is an end view and FIG. 4C is a cross-sectional view throughline 4C-4C of FIG. 4B of CNTs made by the third method of making carbonnanotubes. In FIG. 4B, CNTs or CNT bundles 205 have been grown oncatalytic islands 200 and chemically active as described infra and thenpackaged into filters. As illustrated, growth of CNTs or CNT bundles 205has been stopped prior to adjacent CNTs or CNT bundles touching. In analternative methodology, CNTs or CNTs 205 are allowed to grow to fillthe interior volume of cylindrical substrate 175 with a tangle of CNTsand CNT bundles.

FIG. 5 is a schematic drawing of an apparatus for making carbonnanotubes according to a fourth and fifth method. In FIG. 5, a target300 is placed within a tube 305. Target 300 comprises carbon and one ormore metals such as cobalt (Co), Ni and Fe, which are carbon nanotubecatalysts. A heating element 310 surrounds tube 305. Heating element 310generates a heated zone 315 within tube 305. A cooled collector 320 ispositioned at a downstream end 325 of tube 305 outside of heated zone315. A first laser beam 330A and an optional second laser beam 330Bgenerated by lasers (not shown) are allowed to impinge on target 300from an upstream end 335 of tube 305. An optional tungsten wire or mesh340 is stretched across the diameter of tube 305 between target 300 andcollector 320. Wire or mesh 340 is positioned in heated zone 315. Aninert sweep gas such as argon or helium is introduced into tube 305 fromupstream end 335 of the tube.

In operation, target 300 is heated to between about 1100° C. to about1300° C. The sweep gas may be optionally heated before it enters tube305. In one example, the sweep gas is heated to a temperature betweenabout 400° C. to about 1500° C. Collector 320 is maintained at atemperature between about 50° C. to about 700° C. Laser beam 330A (andoptional laser beam 330B) convert portions of target 300 to mixture ofcarbon vapor and one or more of Co, Ni and Fe metal vapors. The mixtureof carbon vapor and one or more of Co, Ni and Fe metal vapors is sweptby the sweep gas and forms CNTs in heated zone 315 which are thencollected on collector 320. CNTs grow because the group VI or VIIImetals catalyze the growing end of each CNT.

If wire or mesh 340 is present, then the CNTs produced will be longer.They can be as long as the distance between the wire or mesh andcollector 320. When a wire or mesh is used, group VI or VIII metal vaporis not required after initial formation of “seed” CNTs caught on thewire or mesh. Thus, target 300 can be replaced with a target thatcontains only carbon, or target 300 can have an upstream end thatcontains group VI or VIII metals while the bulk of the target containsonly carbon.

The CNTs generated when group VI or VIII metals are present in target300 are predominantly SWNTs. They can have a diameter of about 13.6microns and lengths of about 0.1 micron to about 1000 microns. CNTs arecollected at collector 320 as tangled collection of individual CNTsstuck together in a mat.

In a first alternative version of the third method of forming CNTsdescribed supra, no group VI or VIII metals are present in target 300and wire or mesh 340 is not used so closed spheroid cage fullerenes areproduced instead of CNTs.

In a second alternative version of the third method of forming CNTs(utilizing wire or mesh 340) described supra, the lasers are turned offafter the “seed” CNTs are formed and a hydrocarbon gas added to thesweep gas. Hydrocarbons that may be used include methane, ethane,propane, butane, olefinic, cyclic or aromatic hydrocarbon, or any otherhydrocarbon.

CNTs produced by the third method described supra, often need to bepurified of group VI and VIII metals, amorphous carbon, and othercontaminants. There are many methods known in the art to do this. In oneexample, the mat of CNTs is heated in an acidic oxidizing solution. The“washed” CNTs may be collected in porous polytetrafluoro-ethylenefilters.

A more detailed discussion of formation of CNTs according to the thirdmethod of forming CNTs may be found in United States Patent PublicationUS 2002/0090330 to Smalley et al., filed on Dec. 28, 2001, which ishereby incorporated by reference in its entity.

After the mat of CNTs are formed and cleaned, a chemically active layeris formed on the CNTs as described infra, before being packaged intofilters.

Formation of active layers (either as a chemically active layer on CNTsor as chemically reactive groups on the sidewalls of CNTs) is conductedon CNTs formed on substrates while the CNTs are still on the substrateor in the form of CNT mats.

Many examples forming a chemically active layer on CNTs are known andseveral will now be described.

In a first example an osmium dioxide layer is formed on CNTs by mixingCNTs with osmium tetroxide (OsO₄) in toluene at 25° C. for 2 hours inthe presence of irradiation with light having a wavelength of 254 nmforming OsO2 nanocrystals on the surfaces of the CNTs.

In a second example, a platinum layer is formed on CNTs by pre-treatingthe CNTs with a nitric acid and sulfuric acid mixture at 100° C. for 30minutes, heating the CNTs to about 700° C. for about an hour, reactingthe CNTs with an alcohol solution of hexachloroplatinic acid, and thenheated the CNTs to 700° C., in hydrogen or nitrogen gas. Platinumnanocrystals are formed along the length of the CNTs.

In a third example, Ti, Ni, Au, Pd, Al or Fe layers are formed on CNTsby evaporation of the Ti, Ni, Au, Pd, Al or Fe respectfully onto CNTs.Metal thickness range from about 0.5 nm to about 15 nm. Ti forms Tinano-wires, Ni and Pd form uniform coatings, and Au, Al and Fe form fineparticles on the surface of the CNTs

In a fourth example an SiO_(x) layer is formed on CNTs by immersion inan aqueous solution of about 0.25% polyethylimine, followed by dryingand reaction with an aqueous solution of tetraethoxysilane (TEOS), withultrasonic agitation. After about 96 hours at about 25° C., the SiO_(x)deposition can be terminated. In one example the SiO_(x) layer is about3 nm thick.

Many examples of forming chemically reactive groups on the sidewalls ofCNTs are known and several will now be described.

In a first example, alkyl groups may be attached to the sidewalls ofCNTs by reacting an alkyl lithium or alkyl magnesium (Grignard) reagentwith fluorinated CNTs (see infra for preparation of fluorinated CNTs).In the case of alkyl lithium reagent, the reaction with fluorinated CNTsis performed in hexane for about 5 to about 10 minutes at about 25° C.In the case of alkyl magnesium reagent, the reaction with fluorinatedCNTs is performed in tetrahydrofuran (THF) for 4 hours at about 25° C.Residual fluorine present on the CNTs after reaction with the alkylatingagent can be removed with hydrazine, THF and isopropanol mixture atabout 25° C. for about 30 minutes.

In a second example, fluoro groups may be attached to the sidewalls ofCNTs by reacting CNTs with F₂ gas, diluted with an inert gas such as Heor Ar, at temperatures of about 150° C. to about 60° C. for about 1 to 4hours.

In a third example, aryl groups may be attached to the sidewalls of CNTsby reaction of CNTs with diazonium salts at about 25° C. inacetonitrile, with 5% of the carbon atoms of the CNT being arylated.Alternatively, the reaction can be performed at about 55° C. to about60° C. for about 48 hours in a 5:1 mixture of orthodichlorobenzene andTHF, using an aryl amine and isoamyl nitrite as an in situ source ofdiazonium salt. The aryl groups attached to CNT sidewalls may themselvesbe substituted by using diazonium salts having functional ester, nitro,alkyl, carboxyl, alkyl ether, and acetylenic moieties.

In a fourth example, pyrrolidine groups and substituted pyrrolidinegroups such as alkyl, alkyl ether and aryl substituted pyrrolidine maybe attached to the sidewalls of CNTs by the reaction of CNTs withaldehydes together with N-substituted glycine derivatives at about 130°C. in dimethylformamide (DMF) solvent for about 48 hours.

In a fifth example, hydrogen may be attached to the sidewalls of CNTs byreaction with lithium metal in liquid ammonia, with approximately 10% ofthe carbon atoms of the CNTs being hydrogenated.

In a sixth example, amino groups may be attached to the sidewalls ofCNTs by exposing CNTs to low-pressure ammonia plasma or a low-pressureethylenediamine plasma. Exemplary plasma conditions are a pressure ofabout 0.3 torr, a RF frequency of about 200 kHz, an RF power of about 20watts, for about 1 minute at about 25° C. Amines can also be produced onCNTs by chemical reduction of attached imine groups (described infra)with, for example, sodium cynaoborohydride as the reducing agent.

In a seventh example, aldehyde groups may be attached to the sidewallsof CNTs by exposing CNTs to low-pressure acetaldehyde plasma. Exemplaryplasma conditions are a pressure of about 0.3 torr, a RF frequency ofabout 200 kHz, an RF power of about 20 watts, for about 1 minute atabout 25° C.

In an eighth example, carboxylic groups may be attached to the sidewallsof CNTs by exposing CNTs to a low-pressure acetic acid plasma. Exemplaryplasma conditions are a pressure of about 0.3 torr, a RF frequency ofabout 200 kHz, an RF power of about 20 watts, for about 1 minute atabout 25° C.

In a ninth example, amido groups may be attached to the sidewalls ofCNTs by Amide functionality can be created by an aqueous reaction of thecarboxylic acid derivative of CNTs (see supra) with amines in thepresence of EDC (I-ethyl-3-(dimethylaminopropyl)carbo-di-imide) couplingagent at about 25° C.

In a tenth example, imino groups may be attached to the sidewalls ofCNTs by converting attached aldehyde groups (see supra) to imino groupsby reaction with alkyl amine vapor or ammonia vapor. Also, attachedimino groups can be created by reaction amine functionalized CNTs (seesupra) with ketones or aldehydes. In one example, these reactions areperformed in aqueous solution at about 25° C. at a pH of about 6 toabout 8 over a period of about 24 hours.

In an eleventh example, sulfonic groups may be attached to the sidewallsof CNTs by gas phase sulfonation at about 25° C. for about 2 to about 5minutes with a mixture of about 1% by weight SO₃ in N₂. First anacetaldehyde plasma treatment, or alkane plasma treatment (methane,ethane, propane, hexane, etc) is performed to form a hydrocarbon on thesurface of the CNT.

The next step is to package the CNTs having a chemically active layer orCNTs having chemically reactive groups on their sidewalls into filters.

FIG. 6A is a cross-section view and FIG. 6B is a cross-section viewthrough line 6B-6B of FIG. 6A of a first exemplary chemically activenanotube filter according to the present invention. In FIGS. 6A and 6B,a single substrate 400 having a multiplicity of CNTs 405 has beenpackaged into a filter housing 410 having an inlet 415 and an outlet420. CNTs 405 have either a chemically active layer on the CNTs orchemically reactive groups on the sidewalls of the CNTs.

FIG. 7A is a cross-section view and FIG. 7B is a cross-section viewthrough line 7B-7B of FIG. 7A of a second exemplary chemically activenanotube filter according to the present invention. In FIGS. 7A and 7B,multiple substrates 400A each having a multiplicity of CNTs 405A andmultiple substrates 400B each having a multiplicity of CNTs 405B havebeen packaged into a filter housing 425 having an inlet 430 and anoutlet 435. CNTs 405A have either a chemically active layer on the CNTsor chemically reactive groups on the sidewalls of the CNTs. CNTs 405Bhave either a chemically active layer on the CNTs or chemically reactivegroups on the sidewalls of the CNTs. The chemically active layer orchemically reactive groups may be the same on CNTs 405A and 405B or thechemically active layer or chemically reactive groups on CNTs 405A maybe different from the chemically active layer or chemically reactivegroups on CNTs 405B. Increasing the number of substrate 400A/CNTs 405Aand 400B/CNTs 405B sets allows an increased flow rate of the air or gasbeing filtered and/or increase the lifetime of the filter. By havingdifferent chemically active layer or chemically reactive groups on CNTs405A and 405B, multiple different contaminants can be removed from theair. There may be as many substrate/CNTs combinations, each combinationhaving different chemically active layers or chemically reactive groupsas needed by a particular filtering application.

FIG. 8A is a cross-section view and FIG. 8B is a cross-section viewthrough line 8B-8B of FIG. 8A of a third exemplary chemically activenanotube filter according to the present invention. In FIGS. 8A and 8B,a single hollow cylindrical substrate 440 having a multiplicity of CNTs445 has been packaged into a hollow cylindrical filter housing 450having an inlet 455 and an outlet 460. CNTs 440 have either a chemicallyactive layer on the CNTs or chemically reactive groups on the sidewallsof the CNTs.

FIG. 8C, is an extension of the third exemplary chemically activenanotube filter of FIGS. 8A and 8B. In FIG. 8C, a hollow cylindricalsubstrate 440A having a multiplicity of CNTs 445A and a hollowcylindrical substrate 440B having a multiplicity of CNTs 445B have beenpackaged into a hollow cylindrical filter housing 465 having an inlet470 and an outlet 475. CNTs 445A have either a chemically active layeron the CNTs or chemically reactive groups on the sidewalls of the CNTs.CNTs 445B have either a chemically active layer on the CNTs orchemically reactive groups on the sidewalls of the CNTs. The chemicallyactive layer or chemically reactive groups may be the same on CNTs 445Aand 445B or the chemically active layer or chemically reactive groups onCNTs 445A may be different from the chemically active layer orchemically reactive groups on CNTs 445B. More than two hollowcylindrical substrates each having a multiplicity of CNTs may bearranged in series in a filter housing.

FIG. 9A is a cross-section view and FIG. 9B is a cross-section viewthrough line 9B-9B of FIG. 9A of a fourth exemplary chemically activenanotube filter according to the present invention. In FIGS. 9A and 9B,a first layer 480A of hollow cylindrical substrates 485A having amultiplicity of CNTs 490A, a second layer 480A of hollow cylindricalsubstrates 485B having a multiplicity of CNTs 490C and a third layer480C of hollow cylindrical substrates 485C having a multiplicity of CNTs490C have been packaged into a filter housing 495 having an inletsurface 500 and an outlet surface 505. Layer 480B is positioned betweenlayers 480A and 480C. Individual hollow cylindrical substrates 485A,485B and 485C are positioned so air or gas entering filter housing 495from inlet surface 500 can pass over the multiplicity of respective CNTs490A, 490B and 490C and exit the filter housing from outlet surface 505.A sealant 510 holds individual hollow cylindrical substrates 485A, 485Band 485C in position relative to filter housing 495 and relative to eachother. Spaces between substrates 385A, 485B and 485C are filled withsealant forcing air or gas to pass over CNTs in substrates 485A, 485Band 485C. CNTs 490A, 490B and 490C have either a chemically active layeron the CNTs or chemically reactive groups on the sidewalls of the CNTs.The chemically active layer or chemically reactive groups may be thesame on CNTs 490A, 490B and 490C or the chemically active layer orchemically reactive groups on CNTs 490A, 490B and 490C may be differentfrom one another. While three layers 480A, 480B and 480C areillustrated, as few as one layer and as many as needed layers may bepackaged together in the manner illustrated in FIGS. 9A and 9B.

FIG. 10A is a cross-section view and FIG. 10B is a cross-section viewthrough line 10B-10B of FIG. 10A of a fifth exemplary chemically activenanotube filter according to the present invention. In FIGS. 10A and10B, a first layer 515A of substrates 520A having a multiplicity of CNTs525A, a second layer 515A of substrates 520B having a multiplicity ofCNTs 525C and a third layer 515C of substrates 520C having amultiplicity of CNTs 525C have been packaged into a filter housing 540having an inlet surface 530 and an outlet surface 535. Layer 515B ispositioned between layers 515A and 515C. Individual substrates 520A,520B and 520C are positioned so air or gas entering filter housing 540from inlet surface 530 can pass over the multiplicity of respective CNTs525A, 525B and 525C and exit the filter housing from outlet surface 535.A sealant 545 holds individual substrates 520A, 520B and 520C inposition relative to filter housing 540 and relative to each other. Anoptional thin sheath 550 is positioned around layers 515A, 515B and 515Cto prevent sealant from clogging CNTs on peripheral substrates 520A,520B and 520C. CNTs 525A, 525B and 525C have either a chemically activelayer on the CNTs or chemically reactive groups on the sidewalls of theCNTs. The chemically active layer or chemically reactive groups may bethe same on CNTs 525A, 525B and 525C or the chemically active layer orchemically reactive groups on CNTs 525A, 525B and 525C may be differentfrom one another. While three layers 515A, 515B and 515C areillustrated, as few as one layer and as many as needed layers may bepackaged together in the manner illustrated in FIGS. 10A and 10B.

FIG. 11A is a cross-section view and FIG. 11B is a cross-section viewthrough line 11B-11B of FIG. 11A of a sixth exemplary chemically activenanotube filter according to the present invention. In FIGS. 11A and11B, a first layer 560A of porous walled containers 565A filled withmats of chemically active CNTs, a second layer 560B of porous walledcontainers 565B filled with mats of chemically active CNTs, a thirdlayer 560C of porous walled containers 565C filled with mats ofchemically active CNTs, a fourth layer 560D of porous walled containers565D filled with mats of chemically active CNTs and a fifth layer 560Eof porous walled containers 565E filled with mats of chemically activeCNTs have been packaged into a filter housing 574 having an inletsurface 575 and an outlet surface 580. Layer 560C is the innermost layerand is positioned between layers 560B and 560D. Layer 560B is positionedbetween layers 560A and 560C. Layer 560D is positioned between layers560C and 560E. Air or gas entering filter housing 570 from inlet surface575 passes through each layer 560A, 560B, 560C, 560D and 560E ofrespective porous containers 565A, 565B, 565C, 565D and 525E and exitthe filter housing from outlet surface 580. The CNTs mats in porouscontainers 565A, 565B, 565C, 565D and 565E have either a chemicallyactive layer on the CNTs or chemically reactive groups on the sidewallsof the CNTs. The chemically active layer or chemically reactive groupsmay be the same on CNT mats in porous containers 565A, 565B, 565C, 565Dand 565E or the chemically active layer or chemically reactive groups insome or all of porous containers 565A, 565B, 565C, 565D and 565E 525Cmay be different from one another. While five layers 560A, 560B, 560C,560D and 560E are illustrated, as few as one layer and as many as neededlayers may be packaged together in the manner illustrated in FIGS. 11Aand 11B.

FIG. 12 is a cross-section view of a modified high efficiencyparticulate air (HEPA) filter according to the present invention. InFIG. 12, a filter assembly 580 includes a chemically active CNT filter585 between a HEPA filter 590 and an optional pre-filter 595. Chemicallyactive CNT filter may be a filter as illustrated in FIGS. 9A and 9B, 10Aand 10B, or 11A and 11B and described supra or a mat or set of mats ofchemically active CNTs, the CNTs having either a chemically activecoating or a chemically reactive group on sidewalls of the CNTs.

FIG. 13 is a flowchart of the method of making chemically activenanotube filters according to the present invention. In step 600, asubstrate is provided. In step 605 a catalytic layer is formed on thesubstrate. The catalytic layer may be optionally patterned. In step 610,CNTs are formed on the catalytic layer. As an alternative to steps 600,605 and 610, steps 615 and 620 may be performed. In step 615 a CNTprecursor and CNT catalyst are provided. In step 620, a CNT mat isformed. In step 615, the CNTs on the substrate from step 610 or the CNTsin the CNT mat from step 620 are chemically activated by either forminga reactive layer on the CNTs or forming reactive groups on the sidewallsof the CNTs. In step 630, the substrates with chemically active CNTs orthe chemically active CNT mat are placed in a filter housing.

FIG. 14 is a pictorial representation of an exemplary immersionlithography system incorporating a chemically active nanotube air filteraccording to the present invention. In FIG. 13, an immersion lithographysystem 700 includes a controlled environment chamber 705 and acontroller 710. Contained within controlled environment chamber 705 is afocusing mirror 715, a light source 720, a first focusing lens (or setof lenses) 725, a mask 730, an exposure slit 735, a second focusing lens(or set of lenses) 740, a final focusing lens 745, an immersion head 750and a wafer chuck 755. Immersion head 750 includes a transparent window760, a central chamber portion 765, a surrounding plate portion 770, animmersion liquid inlet 775A and an immersion liquid outlet 775B. Animmersion liquid 785 fills central chamber portion 765 and contacts aphotoresist layer 786 on a top surface 788 of a wafer 790. Plate portion770 is positioned close enough to photoresist layer 786 to form ameniscus 792 under plate portion 770. Window 760 must be transparent tothe wavelength of light selected to expose photoresist layer 786. In oneexample window 760 is transparent to a wavelength of about 190 nm orless.

Focusing mirror 715, light source 720, first focusing lens 725, a mask730, exposure slit 735, second focusing lens 740, final focusing lens745, immersion head 750 are all aligned along an optical axis 800 whichalso defines a Z direction. An X direction is defined as a directionorthogonal to the Z direction and in the plane of the drawing. A Ydirection is defined as a direction orthogonal to both the X and Zdirections. Wafer chuck 755 may be moved in the X and Y directions underthe direction of controller 710 to allow formation of regions of exposedand unexposed photoresist in photoresist layer 786. As XY-stage moves,new portions of photoresist layer 786 are brought into contact withimmersion liquid 785 and previously immersed portions of the photoresistlayer are removed from contact with the immersion liquid. Mask 730 andslit 735 may be moved in the Y direction under the control of controller710 to scan the image (not shown) on mask 730 onto photoresist layer786. In one example, the image on mask 730 is a 1× to a 10×magnification version of the image to be printed and includes one ormultiple integrated circuit chip images.

When exposure is complete, wafer 790 must be removed from controlledenvironment chamber 705 without spilling immersion fluid 785. To thisend, controlled environment chamber 705 also includes a cover plate 795that may be moved to first abut with wafer chuck 755 and then move withthe wafer chuck as the wafer chuck is moved out of position from underimmersion head 750, the cover plate replacing the wafer chuck underimmersion head 750.

Controlled environment chamber 705 includes a supply plenum 805 and anexhaust plenum 810. Air or inert gas is passed from supply plenum 805,through a filter 815, through controlled environment chamber 705 andinto exhaust plenum 810. Because of the high energy and high intensitylight used in immersion lithography system 700, which can cause variousreactions with contaminants in the air or inert gas flowing throughcontrolled environment chamber 705 that can then deposit over toolcomponents and wafer 790 as unwanted polymers, filter 815 containschemically active CNTs having either chemically reactive layers orchemically reactive groups on the sidewalls of the CNTs whosepreparation has been described supra. Any of the filters illustrated inFIGS. 9A and 9B, 10A and 10B, 11A and 11B or 12 and described supra maybe used for filter 815.

While an immersion exposure system has been illustrated in FIG. 14, thepresent invention is applicable to any lithographic system.

Thus, the present invention provides an advanced chemical andparticulate filter for applications requiring extremely low levels ofcontaminants in the filtered air and/or gas streams.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.For example, while multiple methods of forming CNTs have been presented,other methods known in the art may be substituted. Likewise, whilemultiple examples of adding functionality to CNTs has been present,other methods of adding functionality to CNTs known to the art may besubstituted. Additionally, CNTs and CNT bundles may be formed on poroussubstrates, i.e., substrates through which the fluid being filtered maypass, in which case the substrate may be mounted in the filter holderperpendicular to the flow of the fluid through the filter, the CNTs orCNT bundles being on the upstream side of the fluid flow. Therefore, itis intended that the following claims cover all such modifications andchanges as fall within the true spirit and scope of the invention.

1. A method of forming a carbon nanotube filter, comprising: forming amultiplicity of filter elements, each filter element of saidmultiplicity of filter elements formed by: (a) forming a nanoporouslayer on a silicon substrate having a <100> crystal plane orientation byelectrochemical etching of said substrate in a mixture of ethanol andhydrofluoric acid, said nanoporous layer having a pore size of about 300nm; after (a), (b) forming a multiplicity of circular template islandson a top surface of said nanoporous layer by evaporating iron onto a topsurface of said nanoporous layer through a patterned mask; after (b),(c) simultaneously oxidizing said template islands to form amultiplicity of iron oxide catalytic template islands and oxidizing saidnanoporous layer exposed between said template islands to form a silicondioxide layer between said catalytic template islands, after saidoxidizing a thickness of said silicon oxide layer measured perpendicularto a top surface of said substrate is greater than a thickness of saidcatalytic template islands measured perpendicular to said top surface ofsaid substrate; after (c), (d) growing bundles of individual carbonnanotubes on each catalytic template island of said multiplicity ofcatalytic template islands by reacting ethylene with said catalytictemplate islands; and after (d), (e) forming chemically active carbonnanotubes by forming a chemically active layer on said carbon nanotubesor forming chemically reactive groups on sidewalls of said carbonnanotubes; and after forming said multiplicity of filter elementsplacing said filter elements in a filter housing having an inlet and anoutlet along a common axis, said filter elements arranged in a first endof a first stack of filter elements adjacent to said inlet and a firstend of a second stack of filter elements adjacent to said outlet, asecond end of said first stack of filter elements adjacent to a secondend of said second stack, top surfaces of said substrates of said filterelements arranged parallel to said axis and carbon nanotubes of saidfilter elements aligned perpendicular to said axis, filter elements ofsaid first stack of filter elements having different chemically activelayers or chemically reactive groups than filter elements in said secondstack of filter elements.